1. Field of the Invention
The present invention relates to optical systems that provide light beams with polarization that varies along the cross section of the beam.
2. Description of the Related Art
In order to create faster and more sophisticated circuitry, the semiconductor industry continually strives to reduce the size of the circuit elements. The circuits are produced primarily by photolithography. In this process, the circuits are printed onto a semiconductor substrate by exposing a coating of radiation sensitive material to light. The radiation sensitive material is often referred to as a “photoresist” or just resist. Passing the light through a mask, which may consist of a pattern of chrome or other opaque material formed on a transparent substrate, generates the desired circuit pattern. The mask may also be formed by a pattern of higher and lower regions etched into the surface of a transparent substrate, or some combination of the two techniques. Subsequent thermal or chemical processing removes only the exposed or only the unexposed regions of the resist (depending on the material) leaving regions of the substrate bare for further processing which in turn produces the electronic circuit.
Projection exposure systems with a higher numerical aperture and shorter exposure wavelength are desired in order to achieve the highest resolutions and decrease the critical dimension (CD) of features being fabricated. Now the polarization of the exposure light at a reticle and at a wafer can have a substantial impact on imaging. For example, polarization at the reticle (or mask) affects the lithographic performance in several ways. First, the interaction of the illumination with features of the reticle, say, for example, dense lines of chrome, can vary with polarization. The transmission and scattering at a mask then depends on the polarization of the light and features of the mask. Second, reflections at the surfaces of lenses and mirrors are polarization dependent so that apodization and to a lesser degree the wave front of the projection optics (“P.O.”) depend on polarization. Also, the reflection from the surface of the resist depends on polarization, and this too is effectively a polarization dependent apodization. Finally, the rays diffracted from the reticle that are brought back together at the wafer need to interfere to produce an image (also called vector interference). However, only parallel components of the electric field generally can interfere, so the polarization state of each ray at the wafer affects the coherent imaging.
Accordingly, it is increasingly desirable to provide polarized illumination in lithographic systems. Further, as demand for increased resolution and higher NA systems increases, it is increasingly desirable to control polarization across a pupil. A polarization pattern is needed such that different portions of an exposure beam have different polarizations (i.e., different polarization directions). Desired polarization patterns include radial, tangential or other custom polarization patterns.
Heretofore, creating such polarization patterns has been difficult and expensive. One approach provides a mosaic tile structure made up of many birefringent tiles. Each tile can polarize a corresponding section of an exposure beam in a particular direction. In this way, the mosaic of tiles can create a polarization pattern, such as a radial pattern, across a pupil. See, U.S. Pat. No. 6,191,880. Such a mosaic tile structure, however, uses many tiles to provide the polarization pattern. This mosaic of tiles is complicated and difficult to manufacture. Among other things, a sandwich structure may be needed to hold the individual tiles in place across the width of the exposure beam. This is disadvantageous as differential thermal expansion across the mosaic, especially in natural birefringent crystal material, can prevent optical contact and lead to apodization (i.e. undesired intensity variations) at the pupil.
What is needed is a device that can provide polarization patterns, including radial and tangential patterns, without requiring excessively complicated mechanical structure.